Pefloxacin, like other fluoroquinolones, accumulates in macrophages and several other types of nucleated cells (but not in erythrocytes). Upon fractionation of macrophage homogenates by isopycnic centrifugation in sucrose gradients, fluoroquinolones are not found associated with any specific cellular structure. We have compared the activities of pefloxacin and roxithromycin against intracellular Staphylococcus aureus in mouse J774 macrophages. Pefloxacin was significantly more active for equivalent intracellular drug concentrations (i.e. expressed by reference to the respective MICs of the drugs as determined in broth), suggesting differences in intracellular availability and/or capacity of the drugs to express their activity in the intracellular environment. The difference was enhanced by incubating the cells in acidic medium. We have also examined the cellular pharmacokinetics and intracellular distribution of pefloxacin in uninfected and Legionella pneumophila infected guinea pig macrophages. In contrast to uninfected cells from which pefloxacin was quickly released, macrophages infected with legionella retained approximately 20-30% of the accumulated pefloxacin after a 60-min wash-out. Cell fractionation studies indicated that the drug remaining in cells was associated with components of high buoyant density. These fractions also contained [3H] if cells had been incubated with [3H] labelled legionella (by in-vitro exposure to [3H]-thymidine, before phagocytosis). These results suggest that part of the intracellular pefloxacin becomes associated with legionella, or with legionella-containing cytoplasmic structures.